1. Field of the Invention
The invention relates to a process for treating organically polluted water, e.g., for the purpose of producing drinking water, the raw water being intensively mixed in an ozone treatment unit with ozone and then being passed through a filter (fixed-bed reactor) built up from different layers.
2. Description of the Related Art
Generally, the aim of drinking water treatment is not only to remove as completely as possible from the water the loading of concern for human consumption, i.e. contaminants, bacteria etc., but also all those constituents which are considered as nutrients for microorganisms. There is the risk of microorganisms establishing themselves in the distribution network, in particular in branch lines with light flows, and constuning the existing oxygen through respiration during assimilation of these nutrient still remaining in the water. As a hygienic measure, chlorine is conventionally added to the water in order to reduce the bacterial growth. The slogan must therefore be: "Hygiene by nutrient removal". In this manner, the amount of chlorine required can be decreased.
In the process of the Generic type, by no means all of the organic water constituents are completely mineralized by the action of ozone. High-molecular weight compounds are rather only broken down to give smaller units, since at present, the water resources, apart from the natural substances, also contain to an increasing extent anthropogenic high-molecular weight organic substances which in any case are decomposed by the ozone treatment to give low-molecular weight compounds. These constituents thus become nutrients for the bacteria, i.e., assimilable organic substances promoting bacterial growth.
In the process of the generic type, it is further assumed that a multi-layer filter designed as a "fixedbed reactor" is provided downstream of the ozone treatment unit. Such a filter is preferably provided with an upper coarse filter layer comprising filtering charcoal and a fine sand layer beneath this. The filtering charcoal (activated charcoal) primarily has the task of eliminating the residual ozone dissolved in the water, in part by wall catalysis, in part by formation of CO.sub.2. Overall, on the one hand, the filter has a mechanical screening action and on the other hand, however, adsorptive and other boundary-layer processes, chemical reactions and marked biological processes also proceed in the filter. It is known that a more or less thick film, specific to water, of heterotrophic microorganisms, more precisely of the aerobic type, forms on the surface of the filter grains. In the oxygen-rich environment, these microorganisms contribute to further elimination of dissolved nutrients. These are generally communities of various organisms which together form a symbiosis.
The present studies therefore clearly show that even rapid filters having a gravel packing, despite the high filter rates, represent filters working biologically to a greater or lesser extent, the colonization (density) of which probably depends on the raw water quality (Obst/Alexander/Mevius, "Biotechnologie in der Wasseraufbereitung", (Biotechnology in water treatment), R. Olderibourg Verlag Munich Vienna 1990 ISBN 3-486-26104-5, page 95).
The object underlying the invention is to optimize the biological mineralization in the process of the generic type.
In this context, the abovementioned publication, page 85, discloses that, even in nutrient-rich water, the growth of biofilms is limited by the supply of oxygen and organic carbon, the oxygen being the first limiting factor. However, in the process of the generic type, as a result of the intensive ozone treatment in the filter region, sufficient molecular oxygen is present and the nutrient supply, owing to the pretreatment with ozone, is also extensive. Nevertheless, inadequate bacterial growth is frequently observed.